A Wheatstone bridge implemented as a directional coupler is a well-known impedance measurement circuit. It is also known to implement an impedance measurement circuit as a hybrid coupler, i.e., a four-port circuit that has an electrical response such that signals applied to first and second ports provoke a measurement response that is divided between a third, fourth, fifth and sixth output ports. Directional couplers are described in a variety of versions in Straw, Ed., The ARRL Antenna Book 19th Ed., ARRL, Newington, Conn., Copyright 2000, Chapter 27, pp. 27-2 through 27-35. A variety of 50 ohm and 75 ohm directional couplers are available commercially from Mini-Circuits, Brooklyn, N.Y. A monolithic IC solid state RF gain and phase detector circuit, type AD8302, is available from Analog Devices, Norwood, Mass.
Directional couplers are used to measure amplitudes and phases of unknown complex impedance, such as an unknown impedance at an antenna feed point, or an input connection of a transmission line leading to an antenna feed point, for example. One technique for measuring antenna impedance remotely is set forth in R. Barker, “Improved Remote Antenna Impedance Measurement”, QEX magazine, Published by ARRL, Newington, Conn., Jul./Aug. 2004, pages 33-42, the disclosure thereof being incorporated herein by reference.
Handheld portable battery powered antenna analyzers are known in the art. These analyzers typically include an RF frequency generator, a hybrid coupler for coupling the frequency generator to an antenna feed point or transmission line, and a detector for detecting and displaying forward and reflected voltages, for example. Examples of battery-powered analyzers include commercial offerings by AEA, Autek, MFJ, Bird, and the American QRP Club (NJQRP). Many of the low cost battery powered antenna analyzers employ a relatively unstable oscillator as a frequency generator to excite the unknown impedance in order to derive impedance data for measurement. These low cost analyzers augment the unstable oscillator by providing a more accurate time base frequency counter and display. A few analyzers may employ phase lock loop techniques to provide the frequency generator. And, a few others may use direct digital synthesis for the frequency generator.
Handheld vector network analyzers using LC oscillator frequency sources or phase locked loops are known in the art.
LCD displays have been used in these instruments, but have not heretofore used high contrast color transreflective LCD graphic displays needed for effective outdoor daylight operation in the field. In addition, such instruments have not included functionality to characterize and store feed line impedance (compare the Barker QEX article cited above where the technique described is very complex to carry out).
Also, such instruments have not included software-defined graphic user display capabilities for providing the measurement results in a plurality of user-selected formats, including real time Smith Chart, for example, (or Arndt Chart graphing real and complex Rho (complex voltage reflection coefficient, see Walter Maxwell, Reflections II, Transmission Lines and Antennas, WorldRadio Books, Sacramento, Calif., copyright 2001, Chapter 3).